A presensitized lithographic plate has recently been remarkably researched and developed to be used for a Computer to Plate system. A recently proposed presensitized lithographic plate can be attached to a press machine to print an image without development after exposing the plate to light. Another presensitized lithographic plate can be printed on a printing machine after exposing the plate to light on the printing machine. The processing steps can be simplified, and problems of a waste processing solution can be solved by using the recently proposed lithographic plates.
Each of Japanese Patent No. 2,938397, Japanese Patent Provisional Publication No. 9(1997)-127683 and International Publication No. 99/10186 discloses a heat-sensitive presensitized lithographic plate comprising a substrate having a hydrophilic surface and a hydrophilic image-forming layer in which thermoplastic polymer particles are dispersed in matrix (e.g., a hydrophilic resin). The image-recording layer can be heated by converting light (infrared ray) to heat. In the heated image-recording layer, the thermoplastic polymer particles melt and fuse to form a hydrophobic image area on a surface of the hydrophilic image-recording layer. The lithographic plate in which an image has been formed is attached to a cylinder of a printer. The unheated area is removed by applying dampening water and ink to the plate while rotating the cylinder. A conventional developing process in an automatic developing machine can be replaced with the above-described on press development.
Japanese Patent Provisional Publication No. 2000-238452 discloses on press development of a lithographic printing plate having an image-recording layer containing an infrared absorbing agent and micro gel having a surface comprising a functional group to be decomposed with light or thermal energy.
The conventional lithographic plate for on press development has some problems. For example, removal of the unexposed area depends on starting conditions of a press machine. Accordingly, several tens or hundreds sheets should be uselessly printed before obtaining good printed matter. Further, dampening water and rollers for the water is contaminated with a lipophilic component, which has been removed from the plate. Therefore, the rollers should often be washed.
Research Disclosure No. 33302 (January, 1992) discloses a heat-sensitive presensitized lithographic plate having a heat-sensitive layer in which thermoplastic polymer particles are dispersed in a cross-linked hydrophilic resin. Further, each of Japanese Patent Provisional Publication Nos. 7(1995)-1849, 7(1995)-1850, 10(1998)-6468 and 11(1999)-70756 discloses a heat-sensitive presensitized lithographic plate have a hydrophilic layer in which microcapsules (lipophilic particles) are dispersed in a cross-linked hydrophilic binder polymer. Each of the microcapsules contains a lipophilic component. The heat-sensitive presensitized lithographic plate can be heated by light exposure to form a lipophilic image area. The formed surface structure consists of the lipophilic image area and an unexposed hydrophilic non-image area. The surface structure can be used as a surface for lithographic print using dampening water without conducting development (including on press development).
However, the hydrophilic layer provided on a support is not sufficiently hydrophilic. Further, the durability of the hydrophilic layer is also insufficient. Accordingly, the background of an image is gradually contaminated depending on printing conditions.
The conventional hydrophilic layer comprises an acrylamide-hydroxyethyl acrylate copolymer hardened with a methylolmelamine cross-linking agent (described in Japanese Patent Provisional Publication No. 2002-370467), gelatin or polyvinyl alcohol (described in Japanese Patent Provisional Publication No. 11(1999)-95417), or a hydrophilic heat-sensitive polymer comprising a repeating unit containing a group of a quaternary ammonium carboxylate. The conventional hydrophilic layer has an insufficient hydrophilic function of keeping dampening water. Therefore, ink cannot sufficiently be repelled to cause contamination within the hydrophilic non-image area.